Fuel cell stack defrosting

ABSTRACT

A fuel cell power plant comprises a fuel cell stack ( 1 ) constituted by a plurality of fuel cells which perform electric power generation by means of a reaction of hydrogen and oxygen. A controller ( 16 ) determines whether or not moisture inside the fuel cell stack ( 1 ) is frozen, and if the moisture is frozen, the controller ( 16 ) causes the fuel cell stack ( 1 ) to perform intermittent electric power generation via an inverter ( 27 ) while continuing to supply oxygen to the fuel cell stack ( 1 ). The fuel cell stack ( 1 ) generates heat as a result of the electric power generation, whereby moisture is generated in a cathode ( 9 ). During the periods in which electric power generation is not performed, the oxygen which is supplied to the cathode ( 9 ) of the fuel cells scavenges the generated moisture, thereby ensuring the supply of oxygen to the cathode ( 9 ) during electric power generation.

FIELD OF THE INVENTION

This invention relates to the defrosting of ice in the interior of afuel cell stack when the fuel cell stack is operated below freezingpoint.

BACKGROUND OF THE INVENTION

Water exists in various locations in a polymer electrolyte fuel cell(PEFC). During operations of the fuel cell, for example, a polymerelectrolyte membrane is maintained in a damp state. Moreover, pure wateris generated in the cathode of the fuel cell during electric powergeneration. Further, since the fuel cell generates heat during electricpower generation, a cooling water passage is formed in the fuel cell.Hence when the fuel cell is placed in below freezing conditions for along period of time, the moisture in the interior thereof freezes. Inorder to operate the fuel cell in this state, first the interior icemust be defrosted.

JP2000-315514A, published by the Japanese Patent Office in 2000,proposes the use of high temperature fluid heated using the electricpower of a secondary battery to defrost the moisture inside a fuel cell.

JP2000-512068A, published by the Japanese Patent Office in 2000,proposes that electric power generation in the fuel cell be started in afrozen state such that the ice in the interior of the fuel cell isdefrosted by the heat generated during power generation.

SUMMARY OF THE INVENTION

A power plant according to JP2000-315514A is dependent upon thesecondary battery for all types of driving energy such as heating energyand energy required for recirculating high temperature fluid to the fuelcell. As a result, the load on the secondary battery is large and thus alarge-size secondary battery is necessary.

In the power plant according to JP2000-512068A, when power generation isperformed in the fuel cell with all of the interior moisture frozen,water vapor which is generated in the cathode is cooled rapidly due toheat exchange with peripheral members, thereby condensing to form wateror ice. This water or ice blocks the gas passage and gas diffusion layerof the cathode, thereby obstructing the supply of air to the cathode. Inthis state the power generation reaction is insufficient and the amountof generated heat is small, and thus a large amount of time is requiredfor the ice to defrost completely such that the fuel cell can beoperated normally. In order to prevent blockages in the gas passage andgas diffusion layer, power generation must be performed at a low powercurrent value, but in so doing the amount of heat generated by the powergeneration reaction is small, and thus defrosting still requires a largeamount of time.

It is therefore an object of this invention to shorten the start-up timeof a fuel cell stack in a frozen state without expending the electricalpower of a secondary battery.

In order to achieve the above object, this invention provides a fuelcell power plant comprising a fuel cell stack comprising fuel cellswhich generate electric power under a supply of hydrogen and oxygen, amechanism which supplies oxygen to the fuel cell stack, a sensor whichdetects a parameter for determining if moisture in the fuel cell stackis frozen, and a controller.

The controller functions to determine if the moisture in the fuel cellstack is frozen based on the parameter, and cause the fuel cell stack toperform intermittent electric power generation when the moisture in thefuel cell stack is frozen.

This invention also provides a control method of such a fuel cell powerplant that comprises a fuel cell stack comprising fuel cells whichgenerate electric power under a supply of hydrogen and oxygen and amechanism which supplies oxygen to the fuel cell stack. The methodcomprises detecting a parameter for determining if moisture in the fuelcell stack is frozen, determining if moisture in the fuel cell stack isfrozen based on the parameter, and causing the fuel cell stack toperform an intermittent generation of electric power when the moisturein the fuel cell stack is frozen.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell power plant according tothis invention.

FIG. 2 is a flowchart describing a routine for defrosting a fuel cellstack performed by a controller according to this invention.

FIGS. 3A-3C are timing charts describing the variation of a powercurrent, temperature and voltage of a fuel cell of the power plantduring start-up below freezing point.

FIG. 4 is a diagram showing the relationship between the power currentand voltage of the fuel cell.

FIG. 5 is a flowchart describing a routine for controlling hydrogensupply to the fuel cell stack performed by the controller in parallelwith the defrosting routine.

FIG. 6 is a flowchart describing a routine for defrosting a fuel cellstack performed by a controller according to a second embodiment of thisinvention.

FIGS. 7A and 7B are timing charts describing the variation of a powercurrent and voltage of a fuel cell of the power plant during start-upbelow freezing point according to the second embodiment of thisinvention.

FIG. 8 is a flowchart describing a routine for defrosting a fuel cellstack performed by a controller according to a third embodiment of thisinvention.

FIG. 9 is a diagram describing the contents of a power current parametertable stored by the controller according to the third embodiment of thisinvention.

FIG. 10 is a schematic diagram of a fuel cell power plant according to afourth embodiment of this invention.

FIGS. 11A-11C are timing charts describing the variation of a powercurrent, temperature and voltage of a fuel cell of the power plantduring start-up below freezing point according to the fourth embodimentof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a fuel cell power plant forinstallation in a vehicle comprises a fuel cell stack 1. The fuel cellstack 1 is constituted by a large number of fuel cells connected inseries, but for ease of explanation, the fuel cell stack 1 in thedrawings is illustrated with a single fuel cell.

A hydrogen supplying passage 3, an air supplying passage 10, achange-over valve 6, and an outlet 12 are connected to the fuel cellstack 1.

Each of the fuel cells of the fuel cell stack 1 comprises a polymerelectrolyte membrane 25 interposed between an anode 2 and a cathode 9.

A flow control valve 4 is installed in the hydrogen supplying passage 3to control hydrogen supply from a hydrogen tank 26 to the anode 2 ofeach fuel cell. The change-over valve 6 selectively leads anode effluentcontaining surplus hydrogen not used in the power generation reactionwhich is discharged from the anode 2 of each fuel cell to arecirculation passage 7 or an outlet 5. The recirculation passage 7 isconnected to the hydrogen supplying passage 3 via an ejector pump 8which suctions anode effluent in the recirculation passage 7 by using asuction force generated by the flow velocity of hydrogen which passesthrough the ejector pump 8. The outlet 5 opens onto the atmosphere.

The air supplying passage 10 supplies air issued from a blower 11 to thecathode 9 of each fuel cell. The outlet 12 releases cathode effluentcontaining water vapor generated by the power generation reaction andoxygen not used in the power generation reaction which are dischargedfrom the cathode 9 of each fuel cell into the atmosphere.

Electrical wires 13 and 14 for extracting a direct power currentgenerated by the fuel cell are connected to the fuel cell stack 1. Theelectrical wires 13 and 14 are connected to an electrical load 15. Here,the electrical load 15 is a generic term comprising an electric motorused for driving the vehicle, the blower 11, various auxiliary machinerysuch as a pump, a secondary battery and a charging/dischargingcontroller therefor, a vehicle air conditioning device, variouslighting, and other electrical components. Power current consumption inthe electrical load 15 is controlled via an inverter 27.

Operation of the blower 11, switching of the change-over valve 6, andpower current consumption in the electrical load 15 are controlled by acontroller 16.

The controller 16 is constituted by a microcomputer comprising a centralprocessing unit (CPU), read only memory (ROM), random access memory(RAM), and an input/output interface (I/O interface). The controller maybe constituted by a plurality of microcomputers.

When the fuel cell power plant is to be started up below the temperatureat which moisture inside the fuel cell stack 1 freezes, the fuel cellstack 1 must be defrosted. This defrosting can be efficiently realizedin a short time period by having the controller 16 appropriately controlthe power generation load in the fuel cell stack 1 during start-up.

In order to perform this control, the fuel cell power plant comprises atemperature sensor 19 for measuring the temperature of the interior ofthe fuel cell stack 1, a pressure sensor 21 for detecting the pressureof the anode effluent, a volt meter 17 for detecting the terminalvoltage of the fuel cell stack 1, an ammeter 18 for detecting thecurrent consumption of the electrical load 15, an external temperaturesensor 20 for detecting the temperature of the atmosphere Ta, and a mainswitch 28 for commanding start-up of the fuel cell power plant. Thedetected data of each of these sensors are input into the controller 16as signals.

Next, referring to FIG. 2, a routine for defrosting the fuel cell stack1 which is executed by the controller 16 will be described. The fuelcell power plant is started up when a driver of the vehicle switches onthe main switch 28. This routine is executed upon detection of the mainswitch 28 being switched on.

In a step S1, the controller 16 determines whether or not the fuel cellstack 1 is in a frozen state. This determination is performed in orderto judge whether or not there is a likelihood of the supply of air tothe cathode being blocked due to the water vapor generated upon powergeneration turning to water or ice when power generation is performedwith the moisture inside the fuel cell stack 1 in a frozen state. Thisphenomenon becomes more likely to occur as the air temperature falls,and therefore an experiment is performed in advance to determine the airtemperature boundary at which this air supply blocking phenomenonappears. The controller 16 determines that the fuel cell stack 1 is in afrozen state when an atmospheric temperature Ta detected by the externaltemperature sensor 20 is below a predetermined temperature Te set on thebasis of this boundary temperature. If it is determined that the fuelcell stack 1 is in a frozen state, the controller 16 executes theprocessing in steps S3-S9.

If, on the other hand, the atmospheric temperature Ta detected by theexternal temperature sensor 20 is not below the predeterminedtemperature Te, the controller 16 executes start-up processing for thefuel cell power plant at a normal temperature in a step S2, and thenends the routine. Start-up processing for the fuel cell power plant at anormal temperature pertains to prior art bearing no relationship to thisinvention, and hence description thereof has been omitted.

Determination of the frozen state of the fuel cell stack 1 may beperformed on the basis of a temperature T of the fuel cell stack 1detected by the temperature sensor 19 instead of the atmospherictemperature Ta detected by the external temperature sensor 20.

When the fuel cell stack 1 is in a frozen state, the controller 16 firstbegins to operate the blower 11 in a step S3. As a result, hydrogen andair are supplied respectively to the anode 2 and cathode 9 of the fuelcell stack 1.

Next, in a step S4, the controller 16 reads the temperature Tof the fuelcell stack 1 which is detected by the temperature sensor 19.

Next, in a step S5, the controller 16 retrieves a power currentparameter table which is stored in advance in internal memory on thebasis of the temperature T of the fuel cell stack 1 to determine a pulsewidth t1 and pulse interval t2 for power current pulses to be output bythe fuel cell stack 1 in accordance with the temperature T TABLE-1 is anexample of the power current parameter table. TABLE 1 FUEL CELL T1 T2 T3T4 T5 T6 T7 T8 STACK TEMPERATURE T (° C.) PULSE WIDTH t11 t12 t13 t14t15 t16 t17 t18 PULSE t21 t22 t23 t24 t25 t26 t27 t28 INTERVALwhere,T1 < T2 < . . . < T7 < T8,t11 < t12 < . . . < t17 < t18; andt21 > t22 > . . . > t27 > t28.

Referring to TABLE-1, the power current parameter table is characterizedin that the pulse width t1 increases and the pulse interval t2 decreasesas the temperature T rises. Here, the pulse width t1 indicates theduration of a pulse, and the pulse interval t2 indicates an intervalfrom the halting of pulse current output by the fuel cell stack 1 to thestart of the next pulse current output. The controller 16 sets the pulsewidth t1 and pulse interval t2 in accordance with the temperature T fromthe power current parameter table. The power current parameter table isset in advance experientially. In TABLE-1, parameters t1 i, t2 i are setfor each of eight temperatures Ti such that i=1-8, but the value of imay be set arbitrarily. It is also possible to create a numerical modelbased on heat transfer and mass transfer inside the fuel cell stack 1during start-up at low temperatures such that the pulse width t1 andpulse interval t2 are expressed by an equation which is based on thenumerical model.

In a following step S6, the controller 16 controls the inverter 27 suchthat a power current which matches the determined pulse width t1 andpulse interval t2 is output from the fuel cell stack 1. It should benoted that the height of the pulse which is shown in TABLE-1 correspondsto a power current A. The power current A is a fixed value. The settingmethod for the power current A will be described hereinafter.

Next, in a step S7, the controller 16 maintains the controlled state ofthe inverter achieved in the step S6 for a fixed time period.

Next, in a step S8, the controller 16 reads the temperature T of thefuel cell stack 1 detected by the temperature sensor 19 once again.

Next, in a step S9, a determination is made as to whether or not thefuel cell temperature T has reached a defrosting completion temperatureTc of the fuel cell stack 1. The defrosting completion temperature Tc isa temperature at which there is no likelihood of water vapor generatedin the cathode 9 turning to water or ice such that the supply of air tothe cathode 9 is blocked even when the fuel cell stack 1 begins normaloperations.

If, in a step S9, the fuel cell temperature T has not reached thedefrosting completion temperature Tc of the fuel cell stack 1, theprocessing of the steps S5-S9 is repeated until the fuel celltemperature T reaches the defrosting completion temperature Tc. If thefuel cell temperature T has reached the defrosting completiontemperature Tc, the controller 16 ends the routine.

Instead of comparing the fuel cell temperature T with the defrostingcompletion temperature Tc in order to determine the end timing of thedefrosting operation, it is also possible to previously determine thedefrosting operation period according to the atmospheric temperature Tain the step S1, and determine if the elapsed time since the start of thedefrosting operation has reached the defrosting operation period in thestep S9.

Further, it is also possible to monitor the differential pressurebetween the inlet and outlet of the cathode 9 or monitor the outputvoltage of the fuel cell stack 1 to determine the end timing of thedefrosting operation. When the supply of air to the cathode is blockedby ice in the gas passage, the differential pressure between the inletand outlet of the cathode increases and the output voltage of the fuelcell stack 1 falls. By monitoring the differential pressure or theoutput voltage, therefore, it is possible to determine the end timing ofthe defrosting operation without detecting the fuel cell temperature. Inorder to precisely determine the end timing of the defrosting operation,however, it may be required to perform the intermittent power generationwith a large output current and large pulse width.

In any of the above cases, the temperature sensor 19 can be omitted, sothe construction of the fuel cell stack 1 can be simplified.

Following the completion of this defrosting routine, the controller 16executes control for a normal operation.

The supply of air to the fuel cell stack 1 during this defrostingroutine is not performed intermittently, but continuously and at aconstant flow rate. Almost none of the air which is supplied to thecathode 9 during a time period corresponding to the aforementioned pulseinterval t2 is used in the power generation reaction, but insteadfunctions to cause the moisture generated in the cathode 9 by the powergeneration reaction to flow downstream and be discharged from the outlet12 without accumulating in the gas passage and gas diffusion layer whichlie adjacent to the cathode 9. The air which is supplied to the cathode9 has a higher temperature than outside air due to adiabatic compressionperformed by the blower 11, and is generally above freezing point, andis therefore able to perform such a function.

Even if an electrical load is exerted on the fuel cell stack 1, or inother words if, during the period corresponding to the pulse width t1,moisture generated in the cathode 9 accumulates in the gas passage andgas diffusion layer such that the passage of air to the cathode 9 isblocked, the accumulated moisture is pushed downstream by air when noelectrical load is exerted on the fuel cell stack 1, or in other wordsduring the period corresponding to the pulse interval t2, and thus thefuel cell stack 1 is again capable of generating electric power when asubsequent electric load is exerted thereon. This scavenging effect ofthe in-flowing air becomes more striking as the amount of supplied airincreases, and the pulse interval t2 may be decreased as the amount ofsupplied air increases. The amount of air supplied to the fuel cellstack 1 is preferably at least 1.8 times, and more preferably at least 3times the amount of air consumed for pulse current power generation.

As described above, it is desirable that the supply of air to thecathode 9 be continuous rather than intermittent.

As regards the supply of hydrogen to the anode 2, meanwhile, hydrogen isalso not consumed during the period in which the fuel cell stack 1 doesnot generate power, and it is therefore desirable that hydrogen besupplied intermittently in accordance with the pulse current. However,it is difficult to supply hydrogen gas intermittently. Hydrogen may besupplied at an average flow rate which is time integrated with the pulsecurrent, but a high degree of precision is required in the flow ratecontrol of the flow control valve 4.

By having the controller 16 execute a hydrogen supply control routineshown in FIG. 5 during the period of defrosting control of the fuel cellstack 1, or in other words in parallel with the defrosting routine shownin FIG. 2, hydrogen supply to the anode 2 is performed in justproportion.

First, in a step S51, the controller 16 increases the opening of theflow control valve 4.

Next, in a step S52, a determination is made as to whether or not thefuel cell stack 1 is in need of defrosting. This is determined bywhether or not the steps S3-S9 of the defrosting routine in FIG. 2 arecurrently being executed.

If the fuel cell stack 1 is in need of defrosting, the controller 16switches the change-over valve 6 in a step S53 such that the anodeeffluent of the anode 2 flows into the recirculation passage 7 via theejector pump 8, thus forming a closed circuit comprising the ejectorpump 8, the anode 2, the change-over valve 6, and the recirculationpassage 7, through which the anode effluent is recirculated.

Next, in a step S54, the pressure P of the anode effluent detected bythe pressure sensor 21 is read.

Next, in a step S55, a determination is made as to whether or not theanode effluent pressure P exceeds a predetermined pressure P0. Thecontroller 16 waits until the anode effluent pressure P reaches thepredetermined pressure P0, and when the anode effluent pressure Pexceeds the predetermined pressure P0, the controller 16 decreases theopening of the flow control valve 4 in a step S56. During the subsequentperiod in which the fuel cell stack 1 performs pulse current electricpower generation, or in other words in the period corresponding to thepulse width t1, the hydrogen contained in the anode effluent in theclosed circuit is consumed in the anode 2. Through this hydrogenconsumption, the pressure P of the anode effluent falls.

After decreasing the opening of the flow control valve 4, the controller16 reads the anode effluent pressure P once again in a step S57, and ina step S58 compares the anode effluent pressure P with a predeterminedpressure P1. The predetermined pressure P1 is a value for determiningwhether or not the opening of the flow control valve 4 should beincreased again to increase the supply amount of hydrogen from the tank26 in order to compensate for a decrease in the hydrogen concentrationin the anode effluent.

As can be understood from the above explanation, the predeterminedpressure P0 is higher than the predetermined pressure P1.

The controller 16 repeats the processing in the steps S57 and S58 untilthe anode effluent pressure P falls below the predetermined pressure P1in the step S57. When the anode effluent pressure P falls below thepredetermined pressure P1 in the step S57, the controller 16 returns tothe step S51 to increase the opening of the flow control valve 4, andthen repeats the processing of the steps S52-S58.

When the defrosting routine of FIG. 2 is complete, the determinationresult of the step S52 becomes negative, and thus the controller 16 endsthe routine.

According to this routine, hydrogen supply to the anode 2 can beperformed in just proportion during the defrosting routine of FIG. 2.

Next, referring to FIGS. 3A-3C, variation in the pulse current, fuelcell temperature T, and power generation voltage when the fuel cellstack 1 is started up from a frozen state by means of the aforementionedcontrol will be described.

The broken lines in the drawing illustrate characteristics whendefrosting is performed at a constant power generation current a0 as inthe device of JP2000-512068A of the prior art. In this prior art device,a fuel cell stack is started up from a frozen state under a low powercurrent a0 in order to prevent the air supply to the cathode from beingblocked by moisture generated in the cathode during power generation ina frozen state. Directly after the beginning of power generation, theterminal voltage falls slightly below an initial voltage V₀, but sincethe power current a0 is small, the effect thereof is slight. Thetemperature of the fuel cell stack 1 gradually rises due to the heatgenerated by the electric power generation of the fuel cell stack 1.

However, when moisture generated in the cathode accumulates in the gaspassage and gas diffusion layer such that air is prevented from reachingthe cathode, the power generation voltage of the fuel cell stack 1eventually drops, and when the power generation voltage falls below aminimum value Vmin at a time tc, the fuel cell stack 1 becomes incapableof generating power. This zero current state continues briefly in thefuel cell stack 1. In this state, no power generation reaction takesplace, and therefore no water is generated in the cathode. Then, whenthe moisture accumulated in the gas passage and gas diffusion layerdiffuses such that the air supply is able to reach the cathode, the fuelcell stack 1 resumes the power generation reaction, and at a time td theterminal voltage rises above the minimum value Vmin. By suppressing thepower generation current of the fuel cell stack 1 in this conventionaldevice to the low power current a0 in this manner, temperature increasesin the fuel cell stack 1 are extremely slow, as shown in FIG. 3B, andfurthermore, under the low power current a0, a state of power generationincapability may occur as shown in the time period tc−td.

In the fuel cell power plant according to this invention, on the otherhand, the controller 16 refers to a table which is stored in internalmemory in advance on the basis of the fuel cell temperature T atstart-up time to determine the pulse width t1 and pulse interval t2. If,for example, the fuel cell temperature T=T2, the pulse width t1 is setto t12 and the pulse interval t2 is set to t22. The inverter 27 is thencontrolled such that power generation is performed over a fixed timeperiod according to the set pulse width t12 and pulse interval t22. Thepower current A at this time greatly exceeds the power current a0 in theconventional device, and hence the drop in voltage accompanying powergeneration is also large. This large drop in voltage, or in other wordslow power generation efficiency, causes heat generation such that alarger amount of heat can be generated than in the conventional device.As a result, as shown in FIG. 3B, the temperature T of the fuel cellstack 1 rises rapidly.

Since power generation is performed under a large power current, a largeamount of moisture is generated in the cathode 9, and the generatedmoisture begins to block the supply of air to the cathode 9. However,when the voltage falls to the minimum voltage Vmin, the time periodcorresponding to the pulse width t12 elapses such that power generationin the fuel cell stack 1 is halted. Meanwhile, air continues to besupplied through the air supply passage 10 and this flow of air reachesthe cathode 9 inside the fuel cell stack 1 to scavenge the moisturewithin the gas passage and gas diffusion layer and discharge thismoisture through the outlet 12.

As a result, the fuel cell stack 1 returns to a state of powergeneration capability. When the pulse interval t22 elapses, powergeneration by the fuel cell stack 1 resumes. By having the controller 16control the inverter 27 such that pulse-form current output is performedin this manner, the fuel cell stack 1 is heated by the heat generationwhich accompanies the output of the large power current A, and by meansof the scavenging action during the pulse interval t22, accumulatedmoisture in the gas passage and gas diffusion layer is removed.Variation in voltage at this time is illustrated in FIG. 3C.

When, as shown in FIG. 3B, the temperature T of the fuel cell stack 1reaches a predetermined temperature T3 following intermittent powergeneration by the fuel cell stack 1 over a fixed time period, thecontroller 16 refers to the table in TABLE-1 once again to set a newpulse width t13 and pulse interval t23. The newly set pulse width t13 islarger than the previous pulse width t12, and the newly set pulseinterval t23 is smaller than the previous pulse interval t22. This isdue to the fact that, among the moisture generated by the powergeneration reaction in the cathode 9, a smaller proportion condenses orfreezes in the gas passage and gas diffusion layer to block the passageof air into the cathode 9 as the temperature Tof the fuel cell stack 1rises. Since the amount of moisture which accumulates in the gas passageand gas diffusion layer decreases, the time required for removing theaccumulated moisture also decreases.

The controller 16 causes the fuel cell stack 1 to resume intermittentpower generation over a fixed time period in accordance with the newpulse width t13 and pulse interval t23. Since the pulse width t13 islarger than the pulse width t12, the amount of heat generated by powergeneration increases, and as shown in FIG. 3B, the temperature T of thefuel cell stack 1 rises more rapidly. When the temperature T of the fuelcell stack 1 reaches a predetermined temperature T4 after this state hascontinued for a fixed time period, the controller 16 references thetable in TABLE-1 once more to set a new pulse width t14 and pulseinterval t24, and then causes the fuel cell stack 1 to resumeintermittent power generation over a fixed time period under the newsettings.

By performing intermittent power generation while resetting the pulsewidth t1 and pulse interval t2 on the basis of the temperature T of thefuel cell stack 1 at fixed time intervals in this manner, increases inthe temperature T of the fuel cell stack 1 accelerate as shown in FIG.3B. The reason why increases in the temperature T of the fuel cell stack1 pause temporarily at zero degrees centigrade, as shown in FIG. 3B, isthat heat generated by the power generation reaction in the fuel cellstack 1 is applied to compensate for the latent heat generated when icein the gas passage and gas diffusion layer as well as ice existing inthe other part of the fuel cell stack 1 are melted and therefore doesnot contribute to the temperature increases in the fuel cell stack 1 assensible heat.

When the temperature T of the fuel cell stack 1 finally reaches atemperature Te at which normal operations are possible, the shift to anormal operation is determined at the next determination opportunity inthe step S9 in FIG. 2, whereupon the controller 16 ends the routine.

Next, referring to FIG. 4, a method of determining the magnitude ofpower current A will be described. The solid line curve in this drawingillustrates a typical relationship between output current and terminalvoltage in a fuel cell stack, and is known as an I-V curve.

A terminal voltage Vt is a logic value calculated on the basis of anamount of energy discharged by an oxidation reaction of hydrogen. Theactual terminal voltage V divided by the logic value Vt is known as thegeneration efficiency. Of the energy which is discharged in powergeneration, the energy which is not converted into electric power, thatis the energy shown by L1 and L2 in the drawing, is consumed in heatgeneration.

As the output current/increases, the terminal voltage Vdrops, and evenwith the same amount of fuel consumption, the amount of energy which isconverted to heat increases. Voltage decrease is particularly strikingin the high current region Z in the drawing. This is due to the factthat the amount of gas consumed in the reaction increases relative tothe diffusion velocity of the reaction gas, i.e., the hydrogen andoxygen, which diffuses on the electrode surface of the fuel cell stack1, and as a result the velocity of the power generation reaction isdependent on the gas diffusion velocity. A decrease in terminal voltagedue to the velocity of gas diffusion is known as a diffusionoverpotential.

The output current A of the fuel cell stack 1 is set in the vicinity ofthe region Z in which the diffusion overpotential becomes dominant. Theoutput current a0 of the fuel cell stack in a frozen state in theconventional device described in JP2000-512068A is set in the vicinityof region X, and hence the amount of generated heat is small.

By setting the output current in the power current region in which thevoltage decreases rapidly due to a diffusion overpotential which isbased on the characteristic of the fuel cell stack 1, the amount of heatgenerated during power generation increases such that the temperature Tof the fuel cell stack 1 can be raised efficiently.

The relationship between output current I and terminal voltage V is notuniform and differs according to the fuel cell stack. Particularly whenactivity decreases under low temperatures or when a part of the fuelcell stack is frozen, performance deteriorates, as shown by the brokenline curve in the drawing, from the standard characteristic shown by thesolid line curve in the drawing. When the performance of the fuel cellstack 1 deteriorates, it is desirable to change the output current A ina frozen state to the vicinity of region Y.

Instead of setting the output current A as a fixed value, the outputcurrent A may be altered dynamically using the phenomenon in which theterminal voltage V decreases dramatically in the regions Z and Y. Morespecifically, the controller 16 controls the power current value suchthat the voltage falls to a preset minimum voltage Vmin. The minimumvoltage Vmin is set at 0.3 to 0.5 volts.

By having the controller 16 control the inverter 27 such that the outputcurrent A determined in this manner is realized, generation efficiencycan be decreased in respect of the same fuel consumption amount, unlikein a conventional device in which a low power current is steadilyextracted from the fuel cell stack, and thus the amount of generatedheat can be increased. Further, since the pulse width t1 and pulseinterval t2 are reset in accordance with increases in the temperature Tof the fuel cell stack 1, accumulated moisture can be removed withcertainty from the gas passage and gas diffusion layer so that a powergeneration reaction can be surely produced in the fuel cell stack 1.

Next, referring to FIG. 6 and FIGS. 7A, 7B, a second embodiment of thisinvention will be described.

The fuel cell power plant according to this embodiment has an identicalhardware constitution to that of the first embodiment, but the logic forcontrolling the pulse-form output current is different to the firstembodiment.

In this embodiment, the controller 16 executes a defrosting routineshown in FIG. 6 in place of the defrosting routine shown in FIG. 2.

The processing in steps S1-S3 and steps S8, S9 is identical to thedefrosting routine of FIG. 2.

After beginning operation of the blower 11 in the step S3, thecontroller 16 controls the inverter 27 in a step S21 to begin powergeneration in the fuel cell stack 1 under the output current A.

Next, in a step S22, the controller 16 reads the terminal voltage V ofthe fuel cell stack 1 which is detected by the voltmeter 17.

Next, in a step S23, the controller 16 compares the terminal voltage Vwith the preset minimum voltage Vmin and repeats the processing in thesteps S22 and S23 until the terminal voltage V falls below the minimumvoltage Vmin. When the terminal voltage V falls below the minimumvoltage Vmin, power generation in the fuel cell stack 1 is halted for afixed time period in a step S24.

Then, similarly to the defrosting routine in FIG. 2, a determination ismade in the steps S8 and S9 as to whether or not the temperature T ofthe fuel cell stack 1 has reached a temperature Tc at which normaloperations are possible. The processing of the step S21 onwards isrepeated until the temperature T reaches the normal operatingtemperature Tc, and when the temperature T reaches the normal operatingtemperature Tc, the routine ends. Control of the air supply to thecathode 9 is performed in a similar manner to the first embodiment.

Variation in the output current and terminal voltage under the controlaccording to this embodiment is illustrated in FIGS. 7A and 7B. As shownin FIG. 7A, the terminal voltage V of the fuel cell stack 1 declinesrapidly as a result of outputting a pulse current corresponding to theoutput current A, but when moisture accumulates in the gas passage andgas diffusion layer such that the air supply to the cathode 9 isblocked, the terminal voltage V declines further to reach the minimumvoltage Vmin.

When the terminal voltage V of the fuel cell stack 1 falls below theminimum voltage Vmin, the controller 16 stops power generation in thefuel cell stack 1 for a fixed time period in a step S24. This stoppageperiod corresponds to the pulse interval t2 of the first embodiment.Once the fixed time period has elapsed, and if the temperature T of thefuel cell stack 1 has not reached the normal operating temperature Te,power generation in the fuel cell stack 1 is resumed under the outputcurrent A.

In this embodiment, power generation is started and stopped on the basisof decreases in the terminal voltage V rather than by setting the pulsewidth t1, and thus a condition in which power generation is impossibledue to the accumulation of moisture in the gas passage and gas diffusionlayer can be avoided with certainty such that power generation can beperformed throughout the entire period in which power generation ispossible. As a result, the temperature of the fuel cell stack 1 can beraised efficiently.

In this embodiment, the power generation stoppage time period of thestep S24 is set at a fixed value, but by resuming power generation whenthe terminal voltage V of the fuel cell stack 1 returns to the initialvoltage V₀, the temperature of the fuel cell stack 1 can be raised evenmore efficiently.

Next, referring to FIGS. 8 and 9, a third embodiment of this inventionwill be described.

The hardware constitution of the fuel cell power plant in thisembodiment is identical to that of the first embodiment, and only themethod for setting the pulse width t1 and pulse interval t2 differs fromthe first embodiment. More specifically, the controller 16 executes adefrosting routine shown in FIG. 8 in place of the defrosting routine inFIG. 2.

Referring to FIG. 8, in this routine steps S31 and S32 are provided inplace of the steps S4 and S5 of the defrosting routine in FIG. 2. Allother steps are identical to those in the routine in FIG. 2. Thecontroller 16 is installed with a timer for counting elapsed time afterthe main switch is switched on by the driver. The elapsed time after themain switch is switched on is equal to the elapsed time following thebeginning of defrosting of the fuel cell stack 1.

In the step S31, the controller 16 reads the elapsed time t0 after themain switch is switched on. Next, in the step S32, a table having acontent as shown in FIG. 9 which is stored in memory in advance isreferred to on the basis of the elapsed time t0 and the atmospherictemperature Ta in order to determine a corresponding pulse width t1 andpulse interval t2.

Referring to FIG. 9, a plurality of types of table is stored in memoryin advance according to the atmospheric temperature Ta, and thecontroller 16 first retrieves the table corresponding to the atmospherictemperature Ta to determine from the obtained table the pulse width t1and pulse interval t2 which correspond to the elapsed time t0.

Here, since the elapsed time t0 is equal to the defrosting time of thefuel cell stack 1, the temperature T of the fuel cell stack 1 rises asthe elapsed time t0 increases. Hence in the table, the pulse width t1and pulse interval t2 are set to increase and decrease respectively asthe elapsed time t0 increases.

As concerns the atmospheric temperature Ta, meanwhile, the pulse widtht1 and pulse interval t2 are set to decrease and increase respectivelyas the atmospheric temperature Ta falls in respect of an identicalelapsed time t0. This is so that power generation obstruction caused bythe accumulation of moisture in the gas passage and gas diffusion layerat low temperatures can be avoided. By setting the pulse width t1 andpulse interval t2 in accordance with these two parameters, i.e. theelapsed time t0 and the atmospheric temperature Ta, the amount of heatgeneration in the fuel cell stack 1 can be increased toward the upperlimit, and thus the amount of time required for defrosting can beshortened.

Next, referring to FIG. 10 and FIGS. 11A-11C, a fourth embodiment ofthis invention will be described.

Referring to FIG. 10, a fuel cell power plant according to thisembodiment comprises a cooling passage 101 for cooling the fuel cellstack 1 and an electric heater 103 for heating cooling liquid. Thecooling liquid in the cooling passage 101 is pressurized by a pump 105to be circulated to the fuel cell stack 1. The electric heater 103 isprovided on a heating passage 102 which bifurcates from the coolingliquid passage 101. The heater 103 generates heat in response to a powersupply from a secondary battery installed in the vehicle to thereby heatthe cooling liquid which is led from the cooling passage 101 to theheating passage 102. The cooling liquid is then recirculated to thecooling passage 101 through the heating passage 102.

When the main switch of the vehicle is switched on below freezing point,the controller 16 first energizes the electric heater 103 and operatesthe pump 105. As a result, the temperature T of the fuel cell stack 1rises as shown in FIG. 11B.

When the temperature T reaches zero degrees centigrade, the controller16 stops energizing the electric heater 103 and operating the pump 105.Hydrogen and air are then supplied to the fuel cell stack 1 and theinverter 27 is controlled such the fuel cell stack 1 outputs apulse-formed current.

The fuel cell stack 1 performs power generation while held at zerodegrees centigrade, and the latent heat which accompanies the melting ofthe interior ice is compensated for by the heat which is generatedduring power generation. When defrosting is complete and the temperatureT of the fuel cell stack 1 reaches the normal operating temperature Te,the controller 16 stops the intermittent power generation of the fuelcell stack 1 and shifts to normal operations. The procedures in any ofthe first through third embodiments may be applied for this intermittentpower generation.

When the fuel cell power plant of this embodiment is started up belowfreezing point, the fuel cell stack 1 is heated using the electricheater 103 while the temperature T of the fuel cell stack 1 is belowfreezing point, and once the temperature T of the fuel cell stack 1 hasreached freezing point, temperature increases in the fuel cell stack 1are realized by the heat which is generated during the intermittentpower generation of the fuel cell stack 1. When the fuel cell stack 1 iscaused to perform power generation below freezing point, the air supplyto the cathode 9 becomes more likely to be blocked due to moisturegenerated in the cathode 9.

Hence in this embodiment, the heat produced by the electric heater 103and the heat produced by the power generation reaction are separated ata boundary of zero degrees centigrade. The heat energy which is used forheating the fuel cell stack 1 is divided into sensible heat forincreasing the temperature of the fuel cell stack 1 and latent heatwhich is expended in the melting of ice inside the fuel cell stack 1,although generally, latent heat exceeds sensible heat when the fuel cellstack 1 is heated from below freezing point.

The electric heater 103 which is operated by a power supply from thesecondary battery is capable of supplying heat regardless of whether thefuel cell stack 1 is in a frozen state or not. Once the temperature T ofthe fuel cell stack 1 has reached zero degrees centigrade, heating whichis equivalent to the latent heat is performed by the heat generatedduring the intermittent power generation reaction of the fuel cell stack1, and thus the energy consumption of the secondary battery 104 isminimized. Further, by charging the secondary battery 104 by means ofintermittent power generation, the charge amount of the secondarybattery 104 can be increased or a driving power can be supplied toauxiliary machinery.

A large amount of electrical energy must be consumed to increase thetemperature T of the fuel cell stack 1 to the normal operatingtemperature Te using the electric heater 103 alone, but if the electricheater 103 is used only to heat the fuel cell stack 1 to zero degreescentigrade, the power consumption of the electric heater 103 is greatlysuppressed.

Hence according to this embodiment, normal operations can be started ina shorter amount of time than when the fuel cell stack 1 is warmed to astate in which normal operations are possible by defrosting the frozenmoisture therein using only the electric heater 103 or only the powergeneration reaction of the fuel cell stack 1.

Although the boundary temperature is set equal to zero degreescentigrade in this embodiment, the temperature boundary at which the airsupply blocking phenomenon appears is not necessarily zero degreescentigrade. The real temperature boundary is different depending onthermal capacity of fuel cells, temperature and thermal capacity ofpiping around the fuel cells, temperature of gas provided to the fuelcells, etc. So the boundary temperature is preferably determined throughexperiment.

The contents of Tokugan 2002-185889, with a filing date of Jun. 26, 2002in Japan, are hereby incorporated by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings.

INDUSTRIAL FIELD OF APPLICATION

According to this invention as described above, by performing powergeneration intermittently when a fuel cell stack in a frozen state isdefrosted by means of fuel cell power generation, moisture which isgenerated in the cathode during the power generation is scavenged byoxygen supplied while the power generation is halted. As a result, thesupply of oxygen to the cathode is not blocked by the accumulatedmoisture and power generation can be performed by the fuel cell stackunder a large power current even when frozen. Accordingly, when thisinvention is applied to a fuel cell power plant for driving a vehicle, afrozen fuel cell stack can be warmed in a short period of time withoutreceiving an external energy supply.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A fuel cell power plant comprising: a fuel cell stack comprising fuelcells which generate electric power under a supply of hydrogen andoxygen; a mechanism which supplies oxygen to the fuel cell stack; asensor which detects a parameter for determining if moisture in the fuelcell stack is frozen; and a controller functioning to: determine if themoisture in the fuel cell stack is frozen based on the parameter; andcause the fuel cell stack to perform intermittent electric powergeneration when the moisture in the fuel cell stack is frozen.
 2. Thefuel cell power plant as defined in claim 1, wherein the controllerfurther functions to cause oxygen supplying mechanism to continuouslysupply oxygen to the fuel cell stack when causing the fuel cell stack toperform intermittent electric power generation.
 3. The fuel cell powerplant as defined in claim 1, wherein the fuel cell stack generateselectric power in response to a power requirement, the power plantfurther comprises a mechanism which regulates the power requirement, andthe controller further functions to control the regulating mechanism,when the moisture in the fuel cell stack is frozen, to cause the fuelcell stack to perform the intermittent electric power generation.
 4. Thefuel cell power plant as defined in claim 3, wherein the fuel cell stackis electrically connected to an electrical load, and the regulatingmechanism comprises an inverter which regulates power supply to theelectrical load from the fuel cell stack.
 5. The fuel cell power plantas defined in claim 4, 3, wherein the controller further functions tocontrol the regulating mechanism to cause an output current of theintermittent electric power generation by the fuel cell stack tocoincide with a current at which a decrease in an output voltage of thefuel cell stack occurs due to a diffusion overpotential.
 6. The fuelcell stack as defined in claim 4 1, wherein the parameter is one of atemperature of the fuel cell stack and an atmospheric temperature. 7.The fuel cell power plant as defined in claim 1, wherein each of thefuel cells comprises an anode to which hydrogen is supplied and acathode to which oxygen is supplied, the oxygen supplying mechanism isarranged to supply oxygen to the cathode, and the controller furtherfunctions to cause the oxygen supply mechanism, when the fuel cell stackperforms the intermittent electric power generation, to increase anoxygen supply amount to the cathode to not less than 1.8 times of anamount which is required for power generation.
 8. The fuel cell powerplant as defined in claim 1, wherein the power plant further comprises aswitch which starts an operation of the power plant, and the controllerfurther functions to determine if the moisture in the fuel cell stack isfrozen immediately after the switch is turned on.
 9. The fuel cell powerplant as defined in claim 1, wherein the power plant further comprises asensor which detects a temperature of the fuel cell stack, theintermittent electric power generation comprises an output of electriccurrent in the form of pulses, and the controller further functions tovary the width and the interval of the pulses according to thetemperature of the fuel cell stack.
 10. The fuel cell power plant asdefined in claim 9, wherein the controller further functions to increasethe width of the pulses as the temperature of the fuel cell stackincreases.
 11. The fuel cell power plant as defined in claim 9, whereinthe controller further functions to decrease the interval of the pulsesas the temperature of the fuel cell stack increases.
 12. The fuel cellpower plant as defined in claim 1, wherein the power plant furthercomprises a volt meter which detects an output voltage of the fuel cellstack, the controller further functions to cause the fuel cell stack toperform the intermittent electric power generation by causing the fuelcell stack to stop electric power generation, after causing the fuelcell stack to start electric power generation, at a point where theoutput voltage of the fuel cell stack falls below a predeterminedvoltage, and to restart electric power generation when a predeterminedtime has elapsed after electric power generation was stopped.
 13. Thefuel cell power plant as defined in claim 1, wherein the power plantfurther comprises a switch which starts an operation of the power plant,the intermittent electric power generation comprises an output ofelectric current in the form of pulses, and the controller furtherfunctions to count an elapsed time after the switch is turned on andincrease the width of the pulses as the elapsed time increases.
 14. Thefuel cell power plant as defined in claim 13, wherein the power plantfurther comprises a sensor which detects an atmospheric temperature, andthe controller further functions to decrease the width of the pulses asthe atmospheric temperature decreases.
 15. The fuel cell power plant asdefined in claim 1, wherein the power plant further comprises a switchwhich starts an operation of the power plant, the intermittent electricpower generation comprises an output of electric current in the form ofpulses, and the controller further functions to count an elapsed timeafter the switch is turned on, and decrease the interval of the pulsesas the elapsed time increases.
 16. The fuel cell power plant as definedin claim 15, wherein the power plant further comprises a sensor whichdetects an atmospheric temperature, and the controller further functionsto increase the interval of the pulses as the atmospheric temperaturedecreases.
 17. The fuel cell power plant as defined in 1, wherein eachof the fuel cells comprises an anode to which hydrogen is supplied, thefuel cell power plant further comprises a hydrogen supply valve whichregulates hydrogen supply to the anode, a change-over valve whichresupplies an anode effluent discharged from the anode to the anode anda sensor which detects the pressure of the anode effluent, and thecontroller further functions when the moisture in the fuel cell stack isfrozen, to cause the change-over valve to recirculate the anode effluentto the anode and to cause the hydrogen supply valve to maintain thepressure of the anode effluent within a predetermined pressure range.18. The fuel cell power plant as defined in claim 1, wherein the powerplant further comprises a heater which heats the fuel cell stack usingenergy supplied from a source other than the fuel cell stack and asensor which detects a temperature of the fuel cell stack, and thecontroller further functions, when the moisture in the fuel cell stackis frozen, to heat the fuel cell stack using the heater while preventingthe fuel cell stack from performing power generation when thetemperature of the fuel cell stack is lower than a predeterminedtemperature, and to cause the fuel cell stack to perform theintermittent electric power generation when the temperature of the fuelcell stack has reached the predetermined temperature.
 19. A fuel cellpower plant comprising: a fuel cell stack comprising fuel cells whichgenerate electric power under a supply of hydrogen and oxygen; means forsupplying oxygen to the fuel cell stack; means for detecting a parameterfor determining if moisture in the fuel cell stack is frozen; means fordetermining if the moisture in the fuel cell stack is frozen based onthe parameter; and means for causing the fuel cell stack to performintermittent electric power generation when the moisture in the fuelcell stack is frozen.
 20. A control method of a fuel cell power plant,the power plant comprising a fuel cell stack comprising fuel cells whichgenerate electric power under a supply of hydrogen and oxygen and amechanism which supplies oxygen to the fuel cell stack, the methodcomprising: detecting a parameter for determining if moisture in thefuel cell stack is frozen; determining if moisture in the fuel cellstack is frozen based on the parameter; and causing the fuel cell stackto perform intermittent generation of electric power when the moisturein the fuel cell stack is frozen.